Microsoft's Windows OS could play a crucial rule in returning worldwide PC shipments to modest growth next year after multiple years of decline, Gartner said on Monday.

PC shipments could reach around 317 million in 2015, increasing from 308 million units expected to ship this year, the research firm said in a study. Shipments this year are expected to decline by 2.9 percent compared to 2013, which is lower than previous yearly declines.

The "revival" of the PC market will be driven by upgrades of old business PCs with Windows XP, which are no longer supported by Microsoft, said Ranjit Atwal, research director at Gartner. He estimates that roughly 60 million PCs will be upgraded this year.

Businesses are largely upgrading to Windows 7 and avoiding Windows 8, which is viewed more as a tablet OS. Microsoft could release a new OS sometime next year, which could supplant Windows 7 as the OS of choice for businesses. However, it takes time for companies to test and deploy PC OSes, as happened with Windows 7, which took more than a year to find a foothold in businesses.

Counting PCs, tablets and smartphones, Gartner said overall shipments of computing devices are expected to reach 2.4 billion units this year, increasing by 4.2 percent compared to the previous year. Shipments will further increase to 2.6 billion units in 2015.

After the first iPad shipped in 2010, tablets were increasingly adopted as alternative computing devices to PCs. Gartner is projecting tablet shipments to increase to 256 million this year, up from 207 million last year. Tablet shipments will reach 321 million in 2015, overtaking PCs, Gartner said.

Tablets will get cheaper and more functional, Atwal said, adding that these trends will continue to drive adoption in the coming years.

Android will continue to be the dominant OS across devices, according to Gartner. The OS will be installed in 1.17 billion devices shipped this year, an increase of 30 percent. Apple's iOS will receive a boost from the new iPhone due later this year, and the company's iOS and Mac OSes will be in 271 million devices shipped this year, increasing by 15 percent compared to the previous year. Microsoft's Windows desktop OS and Phone OS will be in 333 million devices shipped this year, rising slowly from 326 million the previous year.

But Windows will be in 373.7 million devices shipped in 2015, overtaking the combined shipments of Apple's iOS and Mac OS, which will be in 301.4 million devices, Gartner said. Android will remain the dominant OS, installed in 1.37 billion devices shipped next year.